This article deals with the practical online delay estimation of the remote control system by using an external signal. The external signal can be considered as a special communication loop of the remote control system, and it requires no information from the control system. The delay estimator is based on the adaptive super-twisting algorithm, it accurately estimates the time-varying delay with self-tuned parameters. Moreover, the proposed method also attenuates the effect of the channel inherent noise and deception attack via the network. Finally, this method is validated on a WiFi communication network, and some simulations are given to illustrate the performance and the robustness of the proposed method.